One kind of printing plate comprises a layer of a photo-polymer, which may be a solid, a liquid or a gel. A mask is placed over the photo-polymer. The mask is then illuminated with intense ultraviolet light which shines through clear portions of the mask onto the photo-polymer. Those portions of the photo-polymer which are illuminated through the mask undergo a photo-chemical reaction which hardens the photo-polymer. Areas of the photo-polymer beneath opaque portions of the mask are shaded from the ultraviolet light and are not hardened. After exposure the plate can be developed. This is typically done by washing the plate to remove non-hardened parts of the photo-polymer. Other suitable methods may be used to remove the non-hardened photo-polymer to leave the printing plate with a textured surface useful for printing an image.
Shades of grey in a printed image are produced by a technique called "screening". An area on a printing plate which will print a grey shade is textured with a large number of small screen dots. During printing, each screen dot deposits a small dot of ink on the substrate. No ink is deposited on the substrate in its portions between screen dots. Different shades of grey in the final image are produced by using different sizes and densities of screendots.
The masks used in exposing photo-polymer plates are typically half tone films. Such films have become standard in the printing industry for this purpose. Each portion of a developed halftone film is in one of two states, clear or opaque. Thus both the masks used to make printing plates and the printing plates themselves are "binary" in the sense that they have no intrinsic grey scale. Each part of the mask is either clear or opaque. Each part of the surface of the textured printing plate is either raised, so that it will deposit ink on a substrate in the creation of a printed image, or not raised, so that no ink is deposited on a corresponding portion of the printed image.
A significant problem in preparing flexographic printing plates is that different areas on each flexographic printing plate require different exposures for best results. For example, where the mask has a negative area(a small opaque area surrounded by a large clear area) the printing plate is liable to be overexposed. The small opaque area of the mask should create a corresponding non-printing area on the printing plate. However, light which has entered the photo-polymer through the clear area of the mask can leak into the portion of the photo-polymer under the opaque area. If too much light leaks into the photo-polymer under the opaque area then the photo-polymer under the opaque area may become hardened. This may cause the desired non-printing portion of the printing plate to become "plugged". In extreme cases, the non-printing area may disappear entirely even though the photo-polymer under the small opaque area is shaded from direct illumination.
If a mask has "highlights" (small clear areas in a larger opaque area) then the photo-polymer beneath the clear areas may not receive sufficient exposure. This may result in the absence of desired printing areas, such as screen dots, in the finished printing plate. Many images require a masks which include both negative portions and highlights. It is not possible to provide a single exposure which will properly expose all parts of the photo-polymer plate under such a mask. The exposure latitude for flexographic printing plates is very small since any variation from the optimum exposure will either cause over exposure in negative portions or underexposure in highlights.
The problem of achieving correct exposure of a photo-polymer printing plate is well known in the printing industry. The current state of the art in dealing with such exposure problems is to expose the photo-polymer plate sufficiently to properly expose the highlights and to prevent over exposure in the negative portions of the image by placing small pieces of opaque material on top of the negative portions of the mask during the exposure and moving the opaque material around judiciously during the exposure process. This operation is time consuming and requires a skilled operator to place the pieces of opaque material in appropriate places on top of the mask and to move and remove them at appropriate points in the exposure. Another problem with current techniques is that the resolution with which an image can be imprinted on a photo-polymer printing plate is undesirably limited.
There is a need for a method which can reliably expose a complete photo-polymer printing plate with a single exposure without the intervention of a skilled operator. There is also a need for a method which can prepare flexographic plates having resolutions higher than those readily obtainable with practical current methods.